In order to improve light receiving efficiency of a CMOS image sensor, a backside illuminated image sensor (BIS) in which light is received at the backside, has been suggested.
To implement a backside illuminated image sensor, a process of bonding of a device wafer and a supporting wafer is needed. In the field of a wafer-to-wafer bonding technology, when oxide and silicon are bonded together, voids are generated in the surface of the oxide. The reason this occurs is that the oxide has a rough and hydrophobic surface. Consequently, the oxide has poor stability in bonding process, which results in the voids between wafers.
Voids between the bonded silicon and oxide wafers are undesirable because the voids create transmission degradation and inefficiencies.